update docs

Author: zingale

Docs/source/io.rst

@@ -507,11 +507,12 @@ simulation time, and “0.005” is the level-0 time step. This file
 can be plotted very easily to monitor the time step.
 
 
+.. _sec:global_diag:
 
-Integral Diagnostics
---------------------
+Global Diagnostics
+-------------------
 
-.. index:: castro.sum_interval, integral diagnostics, amr_diag.out, gravity_diag.out, grid_diag.out, species_diag.out
+.. index:: castro.sum_interval, global diagnostics, amr_diag.out, gravity_diag.out, grid_diag.out, species_diag.out
 
 Castro can calculate integrals of quantities on the grid and other
 global quantities and output them to both the screen and to a runtime

Docs/source/performance.rst

@@ -39,6 +39,17 @@ which is controlled by ``amr.subcycling_mode``, as described
 in :ref:`sec:subcycling`.
 
 
+Global Diagnostics
+------------------
+
+.. index:: castro.sum_interval
+
+:ref:`sec:global_diag` are output at regular intervals.  These can
+require reduction operations across all processors, which can be
+expensive for big calculations.  Often we don't need the diagnostics
+every step, so setting ``castro.sum_interval`` to a larger value (like
+``10``) could improve performance.
+
 Metrics
 =======